The present invention relates to an improved heat fixing device for an electrophotographic copying machine.
Heretofore, a heat fixing device used in an electrophotographic copying machine or an electrostatic printing machine or the like was provided with a fixing roll having a heat source in the interior thereof and a pressure roll for pressing a sheet to be fixed against the fixing roll. The outer layer portion of the fixing roll is formed of a non-adhesive non-stick layer such as tetrafluoroethylene or HTV silicone rubber or RTV silicone rubber to prevent offset of the toner image on the sheet while at the same time the surface of the releasing layer is coated with a releasing agent.
Since offset is still produced for both cases when the surface temperature of the fixing roll is too high or too low, the surface temperature of the fixing roll has to be carefully controlled so that no offset is produced. Even in spite of this precaution, a minute amount of offset nonetheless tends to be produced on the fixing roll. The cause of this phenomenon was heretofore considered to be related to such factors as the quantity of toner used, the material and surface temperature of the fixing roll, the pressing force, the feeding rate of the releasing agent, and the time required for the sheet to pass between the fixing roll and the pressure roll. In addition thereto, the adhesive force between the toner and the fixing roll and the cohesive force of the toner were also believed to be significant factors.
Even if these factors are controlled precisely, a minute amount of offset is still produced on the fixing roll. As a result of investigations carried out to determine the cause, it was found that the offset is produced because the fixing roll and/or the pressure roll is charged with static electricity due to friction with the copying sheet when the sheet passes between the fixing roll and the pressure roll. Particularly the surface of the pressure roll, which is formed mostly by an insulating material such as silicone rubber except for a core portion, is charged to a negative high voltage of several thousand to several tens of thousand volts which produces an electric field between the surface of the pressure roll and the core of the fixing roll together with a charge having a reverse polarity flowed from the ground into the core of the fixing roll. As a result, the toner in the electric field is attracted to the side of the fixing roll whereby an offset is produced on the fixing roll.
Yet further, a frictional charge is also produced on a fixing roll having a non-stick layer on its surface but the quantity of charge is small because of the provision of an electroconductive layer constituted by a core inside the surface layer. However, it was found that this had very little effect on offset. Particularly, when the roll core of the fixing roll is coated with tetrafluoroethylene as a heat-resisting agent, the amount of charge produced on the roll surface due to friction is small.
Accordingly, it is considered desirable as a means for preventing charge accumulation to form the surface of the pressure roll as an electroconductive layer and as a means for preventing offset on the fixing roll to reduce the charging potential by providing an electroconductive layer at the inner surface layer of the pressure roll for the fixing roll while at the same time changing the direction of the electric field between the fixing roll and the pressure roll. For forming the surface of the pressure roll as an electroconductive layer, the pressure roll must be manufactured by incorporating an electroconductive material into the elastic body because no presently known elastic materials have sufficiently high electroconductive and heat-withstanding properties. Therefore problems may arise with respect to the durability, abrasion-resisting properties and releasability of the roll.
In consideration of these difficulties, an object of the invention is to provide a heat fixing device in which a grounded electroconductive layer is provided on the inner surface layer of the pressure roll to reduce the charging potential of the surface while at the same time changing the direction of the electric field whereby the offset on the fixing roll is nearly completely prevented.